• 1.导入数据 letter = readtable("J.txt"); letter将成为一个类似excel的表格 通过点属性的方式，可以获取该列数据，下面利用plot绘制出来： plot(letter.X,letter.Y) 结果： axis equal 可以通过该...
一、总体工作流程

二、分类工作流程

1.导入数据

letter = readtable("J.txt");

letter将成为一个类似excel的表格

通过点属性的方式，可以获取该列数据，下面利用plot绘制出来：

plot(letter.X,letter.Y)

结果：

axis equal

可以通过该命令自动调节坐标系的比例：

2.处理数据

letter = readtable("M.txt")
letter.X = 1.5*letter.X;
plot(letter.X,letter.Y)
axis equal

假如表中时间为ms级别，将表中的第一个时间作为起点做差，变成相对时间，最后除以1000，将时间单位变为秒。

letter.Time = letter.Time - letter.Time(1)
letter.Time = letter.Time/1000

plot(letter.Time,letter.X)
plot(letter.Time,letter.Y)

这样做的好处就是将横坐标的数量级拉低。

3.提取特征

Calculating Features

What aspects of these letters could be used to distinguish a J from an M or a V? Instead of using the raw signals, the goal is to compute values that distill the entire signal into simple, useful units of information known as features.

For the letters J and M, a simple feature might be the aspect ratio (the height of the letter relative to the width). A J is likely to be tall and narrow, whereas an M is likely to be more square.
Compared to J and M, a V is quick to write, so the duration of the signal might also be a distinguishing feature.

letter = readtable("M.txt");
letter.X = letter.X*1.5;
letter.Time = (letter.Time - letter.Time(1))/1000
plot(letter.X,letter.Y)
axis equal

dur = letter.Time(end)%%取最后一个时间
aratio = range(letter.Y)/range(letter.X)

其中：

The range function returns the range of values in an array. That is, range(x) is equivalent to max(x)-min(x).

Viewing Features

load featuredata.mat
features

scatter(features.AspectRatio,features.Duration)

gscatter(features.AspectRatio,features.Duration,features.Character)

4.Build a Model

What is a Classification Model?

A classification model is a partitioning of the space of predictor variables into regions. Each region is assigned one of the output classes. In this simple example with two predictor variables, you can visualize these regions in the plane.

There is no single absolute “correct” way to partition the plane into the classes J, M, and V. Different classification algorithms result in different partitions.

下面开始训练模型：准备训练集和测试数据

load featuredata.mat

features

testdata

选用knn模型对训练集进行训练：

knnmodel = fitcknn(features,"Character")

Making Predictions

Having built a model from the data, you can use it to classify new observations. This just requires calculating the features of the new observations and determining which region of the predictor space they are in.

predictions = predict(knnmodel,testdata)

The predict function determines the predicted class of new observations.

predClass = predict(model,newdata)

The inputs are the trained model and a table of observations, with the same predictor variables as were used to train the model. The output is a categorical array of the predicted class for each observation in newdata.
The file featuredata.mat contains a table testdata that has the same variables as features. However, the observations in testdata are not included in features.
Note that testdata contains observations for which the correct class is known (stored in the Character variable). This gives a way to test your model by comparing the classes predicted by the model with the true classes. The predict function will ignore the Character variable when making predictions from the model.

Algorithm Options

By default, fitcknn fits a kNN model with k = 1. That is, the model uses just the single closest known example to classify a given observation. This makes the model sensitive to any outliers in the training data, such as those highlighted in the image above. New observations near the outliers are likely to be misclassified.
You can make the model less sensitive to the specific observations in the training data by increasing the value of k (that is, use the most common class of several neighbors). Often this will improve the model's performance in general. However, how a model performs on any particular test set depends on the specific observations in that set.

knnmodel = fitcknn(features,"Character","NumNeighbors",5)
predictions = predict(knnmodel,testdata)


最后，对比一下预测结果和测试样本：

[predictions,testdata.Character]

6.Evaluate the Model

​load featuredata.mat
testdata
knnmodel = fitcknn(features,"Character","NumNeighbors",5);
predictions = predict(knnmodel,testdata)
iscorrect = predictions == testdata.Character
accuracy = sum(iscorrect)/numel(predictions)

结果：

iswrong = predictions ~= testdata.Character
misclassrate = sum(iswrong)/numel(predictions)
[testdata.Character(iswrong) predictions(iswrong)]

结果：

Accuracy and misclassification rate give a single value for the overall performance of the model, but it can be useful to see a more detailed breakdown of which classes the model confuses. A confusion matrix shows the number of observations for each combination of true and predicted class.

A confusion matrix is commonly visualized by shading the elements according to their value. Often the diagonal elements (the correct classifications) are shaded in one color and the other elements (the incorrect classifications) in another color. You can visualize a confusion matrix by using the confusionchart function.

confusionchart(ytrue,ypred);

where ytrue is a vector of the known classes and ypred is a vector of the predicted classes.

confusionchart(testdata.Character,predictions);

实例

load featuredata13letters.mat
features
testdata

gscatter(features.AspectRatio,features.Duration,features.Character)
xlim([0 10])

knnmodel = fitcknn(features,"Character","NumNeighbors",5);
predictions = predict(knnmodel,testdata);

misclass = sum(predictions ~= testdata.Character)/numel(predictions)
confusionchart(testdata.Character,predictions);

三、导入和预处理数据

1.Creating Datastores

Handwriting Sample Files

Samples of each letter were collected from many different volunteers. Some provided more than one sample of each letter. Each sample was saved in a separate file and all the files were stored in one folder. The file names have the form

user003_B_2.txt
This file would contain the second sample of the letter B written by the volunteer designated “user003”.

Use the datastore function to make a datastore to all files containing the letter M. These files have _M_ in their name and a .txt extension. Store the datastore in a variable called letterds

通过匹配文件名来打开一系列文件，*代表任意字符

letterds = datastore("*_M_*.txt")

You can use the read function to import the data from a file in the datastore.
Using the read function the first time will import the data from the first file. Using it a second time will import the data from the second file, and so on.

data = read(letterds)

plot(data.X,data.Y)

Calling the read function again imports the data from the next file in the datastore.

data = read(letterds)
plot(data.X,data.Y)

The readall function imports the data from all the files in the datastore into a single variable.

data = readall(letterds)
plot(data.X,data.Y)

Custom Preprocessing Functions

Typically you will want to apply a series of preprocessing operations to each sample of your raw data. The first step to automating this procedure is to make a custom function that applies your specific preprocessing operations.

Transformed

Currently, you still need to call your function manually. To automate your data importing and preprocessing, you want your datastore to apply this function whenever the data is read. You can do this with a transformed datastore. The transform function takes a datastore and a function as inputs. It returns a new datastore as output. This transformed datastore applies the given function whenever it imports data.

Normalizing Data

The location of a letter is not important for classifying it. What matters is the shape. A common preprocessing step for many machine learning problems is to normalize the data.
Typical normalizations include shifting by the mean (so that the mean of the shifted data is 0) or shifting and scaling the data into a fixed range (such as [-1, 1]). In the case of the handwritten letters, shifting both the x and y data to have 0 mean will ensure that all the letters are centered around the same point.

letterds = datastore("*_M_*.txt");
data = scale(data);
plot(data.X,data.Y)
axis equal
plot(data.Time,data.Y)
ylabel("Vertical position")
xlabel("Time")

function data = scale(data)
data.Time = (data.Time - data.Time(1))/1000;
data.X = 1.5*data.X;
data.X = data.X - mean(data.X,"omitnan");
data.Y = data.Y - mean(data.Y,"omitnan");
end

preprocds = transform(letterds,@scale)
plot(data.Time,data.Y)

四、特征工程

Statistical Functions

Measures of Central Tendency

Function
Description
mean
Arithmetic mean
median
Median (middle) value
mode
Most frequent value
trimmean
Trimmed mean (mean, excluding outliers)
geomean
Geometric mean
harmean
Harmonic mean

Function
Description
range
Range of values (largest – smallest)
std
Standard deviation
var
Variance
mad
Mean absolute deviation
iqr
Interquartile range (75th percentile minus 25th percentile)
Measures of Shape

Function
Description
skewness
Skewness (third central moment)
kurtosis
Kurtosis (fourth central moment)
moment
Central moment of arbitrary order
1. Calculating Summary Statistic(Quantifying Letter Shapes)

Descriptive Statistics

The handwriting samples have all been shifted so they have zero mean in both horizontal and vertical position. What other statistics could provide information about the shape of the letters? Different letters will have different distributions of points. Statistical measures that describe the shape of these distributions could be useful features.

load sampleletters.mat
plot(b1.Time,b1.X)
hold on
plot(b2.Time,b2.X)
hold off
plot(b1.Time,b1.Y)
hold on
plot(b2.Time,b2.Y)
hold off

aratiob = range(b1.Y)/range(b1.X)
medxb = median(b1.X,"omitnan")
medyb = median(b1.Y,"omitnan")
devyb = mad(b1.Y)

aratiov = range(v1.Y)/range(v1.X)
medxd = median(d1.X,"omitnan")
medyd = median(d1.Y,"omitnan")
devym = mad(m1.Y)

plot(b1.X,b1.Y,b2.X,b2.Y)
axis([-1 1 -1 1])
axis equal
plot(d1.X,d1.Y,d2.X,d2.Y)
axis([-1 1 -1 1])
axis equal

2.Finding Peaks

Local minima and maxima are often important features of a signal. The islocalmin and islocalmax functions take a signal as input and return a logical array the same length as the signal.
idx = islocalmin(x);
The value of idx is true whenever the corresponding value in the signal is a local minimum.

load sampleletters.mat
plot(m1.Time,m1.X)

idxmin = islocalmin(m1.X)
idxmax = islocalmax(m1.X)

plot(m1.Time,m1.X)
hold on
plot(m1.Time(idxmin),m1.X(idxmin),"o")
plot(m1.Time(idxmax),m1.X(idxmax),"s")
hold off

Local minima and maxima are defined by computing the prominence of each value in the signal. The prominence is a measure of how a value compares to the other values around it. You can obtain the prominence value of each point in a signal by obtaining a second output from islocalmin or islocalmax.

[idx,p] = islocalmin(x);

[idx,prom] = islocalmin(m1.X);
plot(m1.Time,prom)

By default, islocalmin and islocalmax find points with any prominence value above 0. This means that a maximum is defined as any point that is larger than the two values on either side of it. For noisy signals you might want to consider only minima and maxima that have a prominence value above a given threshold.

idx = islocalmin(x,"MinProminence",threshvalue)

When choosing a threshold value, note that prominence values can range from 0 to range(x).

idxmin = islocalmin(m1.X,"MinProminence",0.1);
idxmax = islocalmax(m1.X,"MinProminence",0.1);
nnz(idxmin)
sum(idxmin)


3.Computing Derivative

Approximating Velocity

An important aspect of detecting letters written on a tablet is that there is useful information in the rhythm and flow of how the letters are written. To describe the shape of the signals through time, it can be useful to know the velocity of the pen, or, equivalently, the slope of the graph of position through time.
The raw data recorded from the tablet has only position (not velocity) through time, so velocity must be calculated from the raw data. With discrete data points, this means estimating the velocity by using a finite difference approximation v=Δx/Δt

load sampleletters.mat
plot(m2.Time,m2.X)
grid

dX = diff(m2.X);
dT = diff(m2.Time);
dXdT = dX./dT;
plot(m2.Time(1:end-1),dXdT)

maxdx = max(dXdT)
dYdT = diff(m2.Y)./dT;
maxdy = max(dYdT)

Due to limits on the resolution of the data collection procedure, the data contains some repeated values. If the position and the time are both repeated, then the differences are both 0, resulting in a derivative of 0/0 = NaN. However, if the position values are very slightly different, then the derivative will be Inf (nonzero divided by 0).
Note that max ignores NaN but not Inf because Inf is larger than any finite value. However, for this application, both NaN and Inf can be ignored, as they represent repeated data.
You can use the standardizeMissing function to convert a set of values to NaN (or the appropriate missing value for nonnumeric data types).

xclean = standardizeMissing(x,0);
Here, xclean will be the same as x (including any NaNs), but will have NaN wherever x had the value 0.

dYdT = standardizeMissing(dYdT,Inf);
maxdy = max(dYdT)

Try calculating the derivatives of different sample letters. Note that a negative value divided by zero will result in -Inf. You can pass a vector of values to standardizeMissing to deal with multiple missing values at once.

xclean = standardizeMissing(x,[-Inf 0 Inf]);

dYdT = standardizeMissing(dYdT,[-Inf 0 Inf]);
maxdy = max(dYdT)

4.Calculating Correlations

Measuring Similarity

The pair of signals on the left have a significantly different shape to the pair of signals on the right. However, the relationship between the two signals in each pair is similar in both cases: in the blue regions, the upper signal is increasing while the lower signal is decreasing, and vice versa in the yellow regions. Correlation attempts to measure this similarity, regardless of the shape of the signal.

load sampleletters.mat
plot(v2.X,v2.Y,"o-")

For the first half of the letter V, the horizontal and vertical positions have a strong negative linear correlation: when the horizontal position increases, the vertical position decreases proportionally. Similarly, for the second half, the positions have a strong positive correlation: when the horizontal position increases, the vertical position also increases proportionally.
The corr function calculates the linear correlation between variables.

C = corr(x,y);

C = corr(v2.X,v2.Y)

结果：

C = NaN


Because both variables contain missing data, C is NaN. You can use the "Rows" option to specify how to avoid missing values.

C = corr(x,y,"Rows","complete");

C = corr(v2.X,v2.Y,"Rows","complete")

结果：

C = 0.6493


The correlation coefficient is always between -1 and +1.

A coefficient of -1 indicates a perfect negative linear correlation
A coefficient of +1 indicates a perfect positive linear correlation
A coefficient of 0 indicates no linear correlation
In this case, there is only a moderate correlation because the calculation has been performed on the entire signal. It may be more informative to consider the two halves of the signal separately.

M = [v2.X(1:11) v2.Y(1:11) v2.X(12:22) v2.Y(12:22)]

To calculate the correlation between each pair of several variables, you can pass a matrix to the corr function, where each variable is a column of the matrix.

M = [x y z];
C = corr(M);

Cmat = corr(M,"Rows","complete")

注意：进行列与列之间两两相关计算，由于X与Y的相关性和Y与X的相同，因此，该矩阵是一个对称矩阵。

The output Cmat is a 4-by-4 matrix of the coefficients of correlation between each pairwise combination of the columns of M. That is, Cmat(j,k) is the correlation of M(:,j) and M(:,k). The matrix is symmetric because the correlation between x and y is the same as the correlation between y and x. The diagonal elements are always equal to 1, because a variable is always perfectly correlated with itself.

5.Automating Feature Extraction

Custom Preprocessing Functions

Once you have determined the features you want to extract, you will need to apply the appropriate calculations to every sample in your data set. The first step to automating this procedure is to make a custom function that takes the data as input and returns an array of features as output.

Creating a Feature Extraction Function

Currently the script calculates six features for a given letter (stored in the variable letter). The six features are stored in six separate variables.
You can use the table function to combine separate variables into a table.

T = table(x,y,z);

load sampleletters.mat
letter = b1;

aratio = range(letter.Y)/range(letter.X)
idxmin = islocalmin(letter.X,"MinProminence",0.1);
numXmin = nnz(idxmin)
idxmax = islocalmax(letter.Y,"MinProminence",0.1);
numYmax = nnz(idxmax)
dT = diff(letter.Time);
dXdT = diff(letter.X)./dT;
dYdT = diff(letter.Y)./dT;
avgdX = mean(dXdT,"omitnan")
avgdY = mean(dYdT,"omitnan")
corrXY = corr(letter.X,letter.Y,"rows","complete")

featurenames = ["AspectRatio","NumMinX","NumMinY","AvgU","AvgV","CorrXY"];

By default, the table constructed with the table function has default variable names. To make a table with more useful names, use the 'VariableNames' option.

T = table(x,y,z,'VariableNames',["X","Y","Z"]);

Typically you can use either single or double quotes to specify option names. However, because strings can represent data for your table, you need to use single quotes when specifying the 'VariableNames' option.

feat = table(aratio,numXmin,numYmax,avgdX,avgdY,corrXY)

feat = table(aratio,numXmin,numYmax,avgdX,avgdY,corrXY,'VariableNames',featurenames)

At the end of the script, add a local function called extract that takes a single variable, letter, as input and returns a table of features, feat, as output. Copy the code from the beginning of the script and from task 2 to make the body of the function. Test your function by calling it with b2 as input. Store the result in a variable called featB2.

featB2 = extract(b2)

function feat = extract(letter)
aratio = range(letter.Y)/range(letter.X);
idxmin = islocalmin(letter.X,"MinProminence",0.1);
numXmin = nnz(idxmin);
idxmax = islocalmax(letter.Y,"MinProminence",0.1);
numYmax = nnz(idxmax);
dT = diff(letter.Time);
dXdT = diff(letter.X)./dT;
dYdT = diff(letter.Y)./dT;
avgdX = mean(dXdT,"omitnan");
avgdY = mean(dYdT,"omitnan");
corrXY = corr(letter.X,letter.Y,"rows","complete");

featurenames = ["AspectRatio","NumMinX","NumMinY","AvgU","AvgV","CorrXY"];

feat = table(aratio,numXmin,numYmax,avgdX,avgdY,corrXY,'VariableNames',featurenames);
end



Extracting Features from Multiple Data Files

Transformed Datastores

To automate your feature extraction, you want your datastore to apply your extraction function whenever the data is read. As with preprocessing, you can do this with a transformed datastore.

From the raw data, you will typically need to apply both preprocessing and feature extraction functions. You can apply the transform function repeatedly to add any number of transformations to the datastore to the raw data.
The script currently applies the scale function to the files in the datastore letterds. The transformed datastore is stored in the variable preprocds.

letterds = datastore("*.txt");
preprocds = transform(letterds,@scale)

featds = transform(preprocds,@extract)

function data = scale(data)
% Normalize time [0 1]
data.Time = (data.Time - data.Time(1))/(data.Time(end) - data.Time(1));
% Fix aspect ratio
data.X = 1.5*data.X;
% Center X & Y at (0,0)
data.X = data.X - mean(data.X,"omitnan");
data.Y = data.Y - mean(data.Y,"omitnan");
% Scale to have bounding box area = 1
scl = 1/sqrt(range(data.X)*range(data.Y));
data.X = scl*data.X;
data.Y = scl*data.Y;
end

function feat = extract(letter)
% Aspect ratio
aratio = range(letter.Y)/range(letter.X);
% Local max/mins
idxmin = islocalmin(letter.X,"MinProminence",0.1);
numXmin = nnz(idxmin);
idxmax = islocalmax(letter.Y,"MinProminence",0.1);
numYmax = nnz(idxmax);
% Velocity
dT = diff(letter.Time);
dXdT = diff(letter.X)./dT;
dYdT = diff(letter.Y)./dT;
avgdX = mean(dXdT,"omitnan");
avgdY = mean(dYdT,"omitnan");
% Correlation
corrXY = corr(letter.X,letter.Y,"rows","complete");
% Put it all together into a table
featurenames = ["AspectRatio","NumMinX","NumMinY","AvgU","AvgV","CorrXY"];
feat = table(aratio,numXmin,numYmax,avgdX,avgdY,corrXY,'VariableNames',featurenames);
end


Use the readall function to read, preprocess, and extract features from all the data files. Store the result in a variable called data.
There are 12 files and the extract function calculates six features for each. Hence, data should be a 12-by-6 table.
Visualize the imported data by making a scatter plot of AspectRatio on the x-axis and CorrXY on the y-axis.

data = readall(featds)
scatter(data.AspectRatio,data.CorrXY)

The letters that the data represents are given in the data file names, which are of the form usernnn_X_n.txt. Note that the letter name appears between underscore characters (_X_).
You can use the extractBetween function to extract text that occurs between given strings.

extractedtxt = extractBetween(txt,"abc","xyz")

If txt is the string array ["hello abc 123 xyz","abcxyz","xyzabchelloxyzabc"], then extractedtxt will be [" 123 ","","hello"].

knownchar = extractBetween(letterds.Files,"_","_")

For classification problems, you typically want to represent the known label as a categorical variable. You can use the categorical function to convert an array to categorical type.
xcat = categorical(x)
By default, the unique values in x will be used to define the set of categories.

knownchar = categorical(knownchar)

It is convenient to have the known classes associated with the training data. Recall that you can create new variables in a table by assigning to a variable using dot notation.

T.newvar = workspacevar

data.Character = knownchar
gscatter(data.AspectRatio,data.CorrXY,data.Character)

五、分类模型

1.Training a Model

Handwriting Features

The MAT-file letterdata.mat contains the table traindata which represents feature data for 2906 samples of individual letters. There are 25 features, including statistical measures, correlations, and maxima/minima for the position, velocity, and pressure of the pen.

load letterdata.mat
traindata

histogram(traindata.Character)

A boxplot is a simple way to visualize multiple distributions.
boxplot(x,c)
This creates a plot where the boxes represent the distribution of the values of x for each of the classes in c. If the values of x are typically significantly different for one class than another, then x is a feature that can distinguish between those classes. The more features you have that can distinguish different classes, the more likely you are to be able to build an accurate classification model from the full data set.

boxplot(traindata.MADX,traindata.Character)

Use the command classificationLearner to open the Classification Learner app.

Select traindata as the data to use.
The app should correctly detect Character as the response variable to predict.
Choose the default validation option.
Select a model and click the Train button.
Try a few of the standard models with default options. See if you can achieve at least 80% accuracy.
Note that SVMs work on binary classification problems (i.e. where there are only two classes). To make SVMs work on this problem, the app is fitting many SVMs. These models will therefore be slow to train.
Similarly, ensemble methods work by fitting multiple models. These will also be slow to train.

2.Making Predictions

The MAT-file letterdata.mat contains traindata, the table of data used to train the model knnmodel. It also contains testdata which is a table of data (with the same features as traindata) that the model has never seen before.
Recall that you can use the predict function to obtain a model's predictions for new data.

preds = predict(model,newdata)

load letterdata.mat
traindata
knnmodel = fitcknn(traindata,"Character","NumNeighbors",5,"Standardize",true,"DistanceWeight","squaredinverse");
testdata

predLetter = predict(knnmodel,testdata)

misclassrate = sum(predLetter ~= testdata.Character)/numel(predLetter)

The response classes are not always equally distributed in either the training or test data. Loss is a fairer measure of misclassification that incorporates the probability of each class (based on the distribution in the data).

loss(model,testdata)

testloss = loss(knnmodel,testdata)

3.Investigating Misclassifications（Identifying Common Misclassifications）

The Confusion Matrix

For any response class X, you can divide a machine learning model's predictions into four groups:

True positives (green) – predicted to be X and was actually X
True negatives (blue) – predicted to be not X and was actually not X
False positives (yellow) – predicted to be X but was actually not X
False negatives (orange) – predicted to be not X but was actually X
False Negatives

With 26 letters, you will need to enlarge the confusion chart to make the values visible. If you open the plot in a separate figure, you can resize it as large as you like.

The row summary shows the false negative rate for each class. This shows which letters the kNN model has the most difficulty identifying (i.e., the letters the model most often thinks are som

This model has particular difficulty with the letter U, most often mistaking it for M, N, or V.
Some confusions seem reasonable, such as U/V or H/N. Others are more surprising, such as U/K. Having identified misclassifications of interest, you will probably want to look at some the specific data samples to understand what is causing the misclassification.

Identifying Common Misclassifications

When making a confusion chart, you can add information about the false negative and false positive rate for each class by adding row or column summaries, respectively.

confusionchart(...,"RowSummary","row-normalized");

load letterdata.mat
testdata
predLetter
confusionchart(testdata.Character,predLetter);

confusionchart(testdata.Character,predLetter,"RowSummary","row-normalized");

Recall that the Files property of a datastore contains the file names of the original data. Hence, when you import the data and extract the features, you can keep a record of which data file is associated with each observation. The string array testfiles contains the file names for the test data.

Use the logical array falseneg as an index into testfiles to determine the file names of the observations that were incorrectly classified as the letter U. Store the result in a variable called fnfiles.
Similarly, use falseneg as an index into predLetter to determine the associated predicted letters. Store the result in a variable called fnpred.

falseneg = (testdata.Character == "U") & (predLetter ~= "U");

fnfiles = testfiles(falseneg)
fnpred = predLetter(falseneg)


Use the readtable function to import the data in the fourth element of fnfiles into a table called badU. Visualize the letter by plotting Y against X.

badU = readtable(fnfiles(4));
title("Prediction: "+string(fnpred(4)))

4.Investigating Misclassifications（Investigating Features）

load letterdata.mat
traindata
testdata
predLetter

idx = (traindata.Character == "N") | (traindata.Character == "U");
UorN = traindata(idx,:)

idx = (testdata.Character == "U") & (predLetter ~= "U");
fnU = testdata(idx,:)

Categorical variables maintain the full list of possible classes, even when only a subset are present in the data. When examining a subset, it can be useful to redefine the set of possible classes to only those that are in the data. The removecats function removes unused categories.

cmin = removecats(cfull)

UorN.Character = removecats(UorN.Character);

You can use curly braces ({ }) to extract data from a table into an array of a single type.
datamatrix = datatable{1:10,4:6}
This extracts the first 10 elements of variables 4, 5, and 6. If these variables are numeric, datamatrix will be a 10-by-3 double array.

UorNfeat = UorN{:,1:end-1};
fnUfeat = fnU{:,1:end-1};


A parallel coordinates plot shows the value of the features (or “coordinates”) for each observation as a line.
parallelcoords(data)

To compare the feature values of different classes, use the "Group" option.
parallelcoords(data,"Group",classes)

parallelcoords(UorNfeat,"Group",UorN.Character)

Because a parallel coordinates plot is just a line plot, you can add individual observations using the regular plot function.

hold on
plot(fnUfeat(4,:),"k")
hold off


Use the zoom tool to explore the plot. Note that N and U have similar values for many features. Are there any features that help distinguish these letters from each other? A kNN model uses the distance between observations, where the distance is calculated over all the features. Does this explain why N and U are hard to distinguish, even if there are some features that separate them?
When plotting multiple observations by groups, it can be helpful to view the median and a range for each group, rather than every individual observation. You can use the "Quantile" option to do this.

parallelcoords(...,"Quantile",0.2)

parallelcoords(UorNfeat,"Group",UorN.Character,"Quantile",0.2)

5. Improving the Model

Even if your model works well, you will typically want to look for improvements before deploying it for use. Theoretically, you could try to improve your results at any part of the workflow. However, collecting data is typically the most difficult step of the process, which means you often have to work with the data you have.

If you have the option of collecting more data, you can use the insights you have gained so far to inform what new data you need to collect.
In the handwriting example, volunteers were instructed only to write lower-case letters “naturally”. Investigating the data set reveals that there are often discrete groups within a particular letter, such as a block-letter style and a cursive style. This means that two quite different sets of features can represent the same letter.
One way to improve the model would be treat these variants as separate classes. However, this would mean having many more than 26 classes. To train such a model, you would need to collect more samples, instruct the volunteers to write both block and cursive style, and label the collected data accordingly.

If you have the option of collecting more data, you can use the insights you have gained so far to inform what new data you need to collect.
In the handwriting example, volunteers were instructed only to write lower-case letters “naturally”. Investigating the data set reveals that there are often discrete groups within a particular letter, such as a block-letter style and a cursive style. This means that two quite different sets of features can represent the same letter.
One way to improve the model would be treat these variants as separate classes. However, this would mean having many more than 26 classes. To train such a model, you would need to collect more samples, instruct the volunteers to write both block and cursive style, and label the collected data accordingly.

Low accuracy in both your training and testing sets is an indication that your features do not provide enough information to distinguish the different classes. In particular, you might want to look at the data for classes that are frequently confused, to see if there are characteristics that you can capture as new features.
However, too many features can also be a problem. Redundant or irrelevant features often lead to low accuracy and increase the chance of overfitting – when your model is learning the details of the training rather than the broad patterns. A common sign of overfitting is that your model performs well on the training set but not on new data. You can use a feature selection technique to find and remove features that do not significantly add to the performance of your model.
You can also use feature transformation to perform a change of coordinates on your features. With a technique such as Principal Component Analysis (PCA), the transformed features are chosen to minimize redundancy and ordered by how much information they contain.

Low accuracy in both your training and testing sets is an indication that your features do not provide enough information to distinguish the different classes. In particular, you might want to look at the data for classes that are frequently confused, to see if there are characteristics that you can capture as new features.
However, too many features can also be a problem. Redundant or irrelevant features often lead to low accuracy and increase the chance of overfitting – when your model is learning the details of the training rather than the broad patterns. A common sign of overfitting is that your model performs well on the training set but not on new data. You can use a feature selection technique to find and remove features that do not significantly add to the performance of your model.
You can also use feature transformation to perform a change of coordinates on your features. With a technique such as Principal Component Analysis (PCA), the transformed features are chosen to minimize redundancy and ordered by how much information they contain.

The Classification Learner app provides an easy way to experiment with different models. You can also try different options. For example, for kNN models, you can vary the number of neighbors, the weighting of the neighbors based on distance, and the way that distance is defined.
Some classification methods are highly sensitive to the training data, which means you might get very different predictions from different models trained on different subsets of the data. This can be harnessed as a strength by making an ensemble – training a large number of these so-called weak learners on different permutations of the training data and using the distribution of individual predictions to make the final prediction.
For the handwriting example, some pairs of letters (such as N and V) have many similar features and are distinguished by only one or two key features. This means that a distance-based method such as kNN may have difficulty with these pairs. An alternative approach is to use an ensemble approach known as Error-Correcting Output Coding (ECOC) which use multiple models to distinguish between different binary pairs of classes. Hence, one model can distinguish between N and V, while another can distinguish between N and E, and another between E and V, and so on.

When trying to evaluate different models, it is important to have an accurate measure of a model's performance. The simplest, and computationally cheapest, way to do validation is holdout – randomly divide your data into a training set and a testing set. This works for large data sets. However, for many problems, holdout validation can result in the test accuracy being dependent on the specific choice of test data.
You can use k-fold cross-validation to get a more accurate estimate of performance. In this approach, multiple models are trained and tested, each on a different division of the data. The reported accuracy is the average from the different models.
Accuracy is only one simple measure of the model's performance. It is also important to consider the confusion matrix, false negative rates, and false positive rates. Furthermore, the practical impact of a false negative may be significantly different to that of a false positive. For example, a false positive medical diagnosis may cause stress and expense, but a false negative could be fatal. In these cases, you can incorporate a cost matrix into the calculation of a model's loss.


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• 机器学习（1）--线性回归和多项式拟合机器学习（2）逻辑回归 （数学推导及代码实现）机器学习（3）softmax实现Fashion-MNIST分类1.导入MNIST的数据在开始时我们需要导入MNIST的数据，MNIST的数据可以从github上进行...
机器学习（1）--线性回归和多项式拟合机器学习（2）逻辑回归 （数学推导及代码实现）机器学习（3）softmax实现Fashion-MNIST分类1.导入MNIST的数据在开始时我们需要导入MNIST的数据，MNIST的数据可以从github上进行下载Fashion-MNIST数据集，MNIST有四个数据集，分别如下：train-images-idx3-ubyte.gz:    training set images (9912422 bytes)train-labels-idx1-ubyte.gz:   training set labels (28881 bytes)t10k-images-idx3-ubyte.gz:   test set images (1648877 bytes)t10k-labels-idx1-ubyte.gz:   test set labels (4542 bytes)前两个数据集是训练集，分别是训练集的图片和标签，后两个数据集是测试集，分别是测试集的图片和标签。一共有60000张训练集以及10000张测试集。共10种标签，以0~9数字形式给出，对应的描述如下：在第一个数据集中，数据是以2进制给出，其中前128位不包含图片信息，它是关于数据集的一些说明。在图片数据集中（包括数据集和训练集），前面有4个信息，都是由32位二进制数表示，第一个信息是magic number图片位2049，第二个信息是样本数，第三个是行数，第四个是列数。描述如下：其中训练集样本为60000个，测试集样本为10000个。标签数据集，包含两个头文件信息：第一个是magic number，第二个是样本个数。下面是matlab导入图片和标签的函数。function images1=loadImage()
filename="train-images-idx3-ubyte";%将下载的数据解解压后去掉后缀名得到。
fid=fopen(filename,"rb");%打开训练集的图片数据集
images1=A(17:length(A));%从17位开始是图片信息
num=A(5:8);%第二个头文件，样本个数
row=A(9:12);%第三个头文件，行数
col=A(13:16);%第四个头文件，列数
%下面的操作是将32位二进制数转化为十进制
num=dec2hex(num);
row=dec2hex(row);
col=dec2hex(col);
num1=[];
row1=[];
col1=[];
for i=1:4
num1=[num1 num(i,:)];
row1=[row1,row(i,:)];
col1=[col1 col(i,:)];
end
num1=hex2dec(num1);
col1=hex2dec(col1);
row1=hex2dec(row1);
%将图片数据转化为矩阵形式。
images1=reshape(images1,row1,col1,num1);
images1=permute(images1,[2,1,3]);
fclose(fid);
end为了验证转化的是否正确可以利用imwrite函数将矩阵转化为图片看看是否是数字如取第十个矩阵a=images1(:,:,50);imwrite(uint8(a),'test.tif')%输出文件名为test.tif。%imwrite(mat2gray(a), 'test.tif');上述得到的数据是  ，需要转化为  的形式，以便带入模型中学习，并且需要将数据进行归一化处理function [x,x_test,y,y_test]=getData()
%数据文件同函数在同一目录下
train_imgs="train-images-idx3-ubyte";      %训练集
train_labels="train-labels-idx1-ubyte";    %训练集标签
test_imgs="t10k-images-idx3-ubyte";        %测试集
test_labels="t10k-labels-idx1-ubyte";      %测试集标签

x=reshape(x,28*28,60000);
x=x/255.0;  %归一化处理
x=x.';      %转置

x_test=reshape(x_test,28*28,10000);
x_test=x_test/255.0;
x_test=x_test.';
end导入标签：function labels1=loadLabel()
filename="train-labels-idx1-ubyte";
fid=fopen(filename,"rb");
labels1=A(9:length(A));
fclose(fid);
end2.softmax分类2.1softmax回归模型[1]转化为矩阵形式：假设一共有  个样本，每个样本有  个特征，  个标签则：     为输入样本    权重      偏置2.2softmax损失函数softmax使用交叉熵来计算损失函数其中  表示样本总数，  表示第  个样本，  表示第  个输出。 表示第  个样本的真实标签。只有样本  标签为  时，  ，其他标签位为0；损失函数可以表示为：对该损失函数进行求导，由于权重是个矩阵，所以涉及到向量对矩阵的求导。这里重点是  的求导， 其中 ，向量对矩阵的求导就是让向量对矩阵的每一行（或列，这里是对行求导）分别求导，对第  行求导时：对第  行以外的行求导时：又有：则：假设  则：对于每个样本其标签中只有一个为1，其余为0，则当k标签为1时（其中  ）：其中  ，且只有第k个元素为1其余为0；转化为矩阵形式得：类似得对b求导得：matlab实现：%主函数
function [W,b]=softmax(x,y,p,x_test,y_test)
len=length(y);
%将标签表示为热编码得形式
y1=zeros(len,10);
% [m,~]=size(x);
for i=1:len
y1(i,y(i)+1)=1;
end
[m,n]=size(x);
num_iter=1000;
alpha=0.1;
W=ones(n,p);
b=zeros(1,p);
for i=1:num_iter
[Wal,bal]=loss(x,y,W,b);
W=W-alpha*(Wal)/m;
b=b-alpha*(bal)/m;
disp("准确率为："+test(x_test,y_test,W,b));
end
end

%模型函数
function X=net(X)
[m,~]=size(X);
vv=zeros(m,1);
for i=1:m
[v,~]=max(X(i,:));
vv(i,1)=v;
end
X=exp(X-vv);
total=sum(X,2);
X=X./total;
end

%损失函数
function Wal=loss(X,y1,W,b)
y_=net(X*W+b);
[m,~]=size(X);

cost=y1-y_;
Wal=X.'*(y_);
b=sum(cost);
end

%测试函数
function p=test(X,y,W,b)
X=X/255;
y_hat=net(X*W+b);
[~,index]=max(y_hat,[],2);
sum=0;
for i=1:length(y)
if index(i)==y(i)+1
sum=sum+1;
end
end
p=sum/length(y);
end3.数值得上溢和下溢在使用softmax时，分母和分子都用到了  的指数次方，如果数值过大，计算机在处理时会出现溢出，从而得到NaN的数值，导致在进行梯度计算时出现错误，为防止计算机发生溢出，可将每个样本减去该样本的最大特征值，这样可防止数值过大，同时保证分母有一个数为1,不会发生下溢的情况。参考^http://zh.d2l.ai/chapter_deep-learning-basics/softmax-regression.html
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• MATLAB官网入门教程项目描述分类工作流程导入数据1.导入数据2.查看变量处理数据1.矫正单位2.时间归一化特征提取1.分析提取特征2.查看特征建立模型1.创建模型2.做出预测算法选项评估模型1.预测值与真值进行比较2.计算...
MATLAB官方入门教程项目描述分类工作流程导入数据1.导入数据2.查看变量处理数据1.矫正单位2.时间归一化特征提取1.分析提取特征2.查看特征建立模型1.创建模型2.做出预测算法选项评估模型1.预测值与真值进行比较2.计算精确度与误分类度混淆矩阵导入和预处理数据多文本操作1.创建数据存储2.读取数据添加数据转换1.执行数据预处理2.转换后的数据存储3.规范化数据特征工程统计特性形状量化
项目描述

分类工作流程
导入数据
1.导入数据
%data=readtable('myfile.xlsx')

%导入数据
letter='J.txt'

2.查看变量
利用引用的方式查看变量
%x = mytable.Xdata;
%y = mytable.Ydata;

%查看letter的X和Y数据，然后可视化
plot(letter.X,letter.Y)

axis equal 横纵等宽

处理数据
1.矫正单位

什么意思没懂： 我的理解是写的时候，水平距离写一个单位，等于实际测量时的1.5个单位
letter = readtable("M.txt")
letter.X=letter.X*1.5;
plot(letter.X,letter.Y)
axis equal

2.时间归一化

没懂： 我的理解是这个时间值，是写一个字母记录的时间数据，左边的图表示的是任一起始时间点开始写完一个字母所用的时间，右边的图表示，（归一化操作）就是都假定从0开始写完一个字母所用的时间，
可以看到纵坐标表示的是字母书写时垂直坐标的变化
letter = readtable("M.txt")
letter.X = 1.5*letter.X;
plot(letter.X,letter.Y)
axis equal
letter.Time = letter.Time - letter.Time(1) %这里表示归一化，即结果只是每个字母持续的时间
letter.Time = letter.Time/1000 %这里表示换算到毫秒

特征提取
1.分析提取特征
分析有哪些可以利用的特征

这里注意到是长宽比，因为写的时候相同字母可以有填满整个数控板，或者只在数控板的一小块区域书写，所以我认为不能单纯的用长度和宽度来衡量，因为没有考虑尺度的问题
%计算持续时间
dur=letter.Time(end)

%计算横纵比
%利用range函数，range函数返回的向量x的取值范围，range(x)=max(x)-min(X),返回的是标量
aratio=range(letter.Y)/range(letter.X)

2.查看特征
给定数据包含三个变量，两个特征一个标签，470个收集的数据（来自不同的人）
load featuredata.mat
features %将载入的数据重命名为features
scatter(features.AspectRatio,features.Duration) %画散点图

scattter显示的不是很好，利用gscatter分组显示
gscatter(features.AspectRatio,features.Duration,features.Character)
根据标签来利用不同的颜色进行显示

建立模型

注意没有绝对准确的能够将所有类别分清楚的方法
1.创建模型
load featuredata.mat
features
testdata
knnmodel=fitcknn(features,"Character")
%features为导入的表的数据，Character表示表中的标签，（即响应变量，即想让模型预测的类别），范数的输出就是一个拟合好的模型
%注意这里直接写Character而不是features.Character

2.做出预测

%predClass = predict(model,newdata)
%输入是一个训练好的模型，以及要预测结果的测试集，注意这个测试集具有与训练集相同的响应变量
%输出是对每个测试样本的预测结果，组成一个数组
%给定的测试集中包含正确的分类结果，但是predict函数在预测时会忽略正确的变量，从而进行预测

predictions=predict(knnmodel,testdata)

算法选项

没懂
knnmodel = fitcknn(features,"Character","NumNeighbors",5)
predictions = predict(knnmodel,testdata)

评估模型
1.预测值与真值进行比较
load featuredata.mat
testdata
knnmodel = fitcknn(features,"Character","NumNeighbors",5);
predictions = predict(knnmodel,testdata)

% 进行评估
iscorrect=(predictions==testdata.Character)

2.计算精确度与误分类度
精确度
iscorrect=(predictions==testdata.Character)
accuracy=sum(iscorrect)/numel(predictions)

误分类度
misclassrate=sum(predictions~=testdata.Character)/numel(predictions)

混淆矩阵
混淆矩阵可视化准确性和误分类性
利用confusionchart函数可视化
confusionchart(ytrue,ypred)
yture 表示已知的真值标签向量
ypred 表示预测标签（结果）向量

confusionchart(testdata.Character,predictions

导入和预处理数据
多文本操作
有一个文件夹存储所有的数据，每个样本对应一个字母文件

1.创建数据存储
利用datastore函数
letterds=datastore("*_M_*.txt") %这里使用模糊处理

2.读取数据
data=read(letterds) %data是一个表

data=readall(letterds) %相当于把所有文件中的数据都综合成了一个表
plot(data.X,data.Y)

添加数据转换
1.执行数据预处理

letterds = datastore("*_M_*.txt");
data = scale(data);
plot(data.X,data.Y)
axis equal
plot(data.Time,data.Y)
ylabel("Vertical position")
xlabel("Time")

function data=scale(data)
data.Time=(data.Time-data.Time(1))/1000;
data.X=1.5*data.X;
end

2.转换后的数据存储

没懂
preprocds=transform(letterds,@scale) %直接对数据存储进行处理

%读取经处理后的所有数据
plot(data.Time,data.Y)

3.规范化数据

function data=scale(data)
data.Time=(data.Time-data.Time(1))/1000;
data.X=1.5*data.X;
data.X=data.X-mean(data.X,"omitnan"); %控制选项没懂
data.Y=data.Y-mean(data.Y,"omitnan");
end

特征工程
统计特性

形状量化


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• Matlab机器学习App之Regression Learner使用笔记目录软件与数据准备Regression Learner具体使用合理的创建标题，有助于目录的生成如何改变文本的样式插入链接与图片如何插入一段漂亮的代码片生成一个适合你的列表...
Matlab机器学习App之Regression Learner回归分析与模型导出应用目录软件与数据准备Regression Learner具体使用最后
原创作品，转载请注明出处，谢谢~
目录
软件与数据准备

我之前用的是matlabR2016b，里面没有regression learner这个App，现在使用的是Matlab R2019b，如果你的matlab没有的话可以看看是不是版本问题
准备需要进行回归分析的数据，我这里是采用的工作空间中的数据

Regression Learner具体使用

打开App新建会话

在这里我们可以选择自己的样本数据，即gauss_pitch这个矩阵，并使用列作为变量，第三列因变量作为我们的响应，前两列作为预测变量，右侧的验证可以根据需求选择交叉验证折数，或者选择留出法验证（一部分作为训练，另一部分用作验证）
我这里全部采用的如图所示的默认

然后就可以开始我们的会话啦！
选择自己合适的模型，开始训练！

这里我们可以选择不同的模型对我们的数据进行训练
我这里选择了高斯过程回归的平方指数模型进行训练！

这里可以非常直观的看到我们训练的记录以及训练的误差，时间等信息
训练结果分析
我们可以选择响应图，预测实际图等看模型训练的效果并分析

比如这里的响应图，可以根据需要勾选是否显示误差等，X轴的坐标信息也可以更改
导出训练模型并进一步处理
这里我遇到一个小问题，因为我的训练数据是三维的，有两个自变量和一个响应，所以我想要画出样本数据的散点图的同时，利用高斯回归模型画出拟合的三维曲面，这就利用到app导出模型的功能。

可以选择是否导出训练数据，这里我选择第一项直接导出模型。
我这里只用到了trainedModel的predictFcn函数，这里附上写的一段代码

X1 = -100:0.1:100;
X2 = 0:0.001:1.5;
[X1FIT,X2FIT] = meshgrid(X1,X2);
[m, n] = size(X1FIT);

figure(3);
scatter3(gauss_pitch(:,1),gauss_pitch(:,2),gauss_pitch(:,3),15,'b','filled');
hold on;
Y = zeros(m,n);
for num = 1:n
X = [X1FIT(:,num), X2FIT(:,num)];
Y(:,num) = trainedModel.predictFcn(X);
end
mesh(X1FIT, X2FIT, Y);

X1和X2是划分的我感兴趣的范围，希望通过高斯模型绘制曲面的范围，meshgrid的作用就是把X1和X2转变为一个m*n维的矩阵，因此通过for循环，利用模型给预测响应值Y赋值，scatter3绘制出样本数据的散点图；
predictFcn可以根据待预测数据的输入，预测响应，这里我们的输入是X1和X2的范围；
最后，mesh，模型预测的曲面就绘制出来了！

最后
文章中存在的问题欢迎交流指正，如果有更好的使用方法或者使用心得也希望大佬们提出，希望这个笔记也能对大家有所帮助，peace & love~


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